feat: make the Inertia class macroable

[filipebraida]

🔗 Linked issue

Resolves the feature request discussed in adonisjs/core#5121.

❓ Type of change

• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Makes the Inertia class extensible through the standard AdonisJS extension
pattern, in two parts:

1. Make Inertia extend Macroable

Packages can register methods with Inertia.macro() / Inertia.getter()
instead of patching Inertia.prototype — same pattern as Request,
Response, BriskRoute, etc. @poppinss/macroable is added as a dependency
(already used across AdonisJS core packages).

2. Type ctx.inertia against the package entrypoint

The HttpContext augmentation in inertia_middleware.ts now imports the
Inertia type from the package entrypoint instead of the internal module
path. Without this, a third-party declare module '@adonisjs/inertia'
augmentation silently no-ops in TS programs that never import the entrypoint
themselves (e.g. a Tuyau client tsconfig that pulls controllers in through the
generated registry):

error TS2664: Invalid module name in augmentation, module '@adonisjs/inertia' cannot be found.
error TS2339: Property 'modal' does not exist on type 'Inertia'.

With the entrypoint import, any program that type-checks ctx.inertia also
loads the entrypoint declarations, so the interface → class merge always
attaches. The import is type-only, so the compiled middleware gains no runtime
import.

Real-world validation

I migrated adonis-inertia-modal
to this branch — diff:
main...feat/inertia-macroable

• The provider now registers modal() via Inertia.macro(); the package's
  test suite (116 tests, provider booted for real) passes against this branch.
• The type augmentation workaround (force-loading the barrel with a type-only
  import kept alive by a dummy marker export) is deleted entirely, and both
  the server tsconfig and the Tuyau-style client tsconfig of a real app
  type-check inertia.modal(...) correctly.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.